One method of setting the collimation of a beam of coherent light is to input a Gaussian cross-section light beam, such as that from a laser, and then physically to adjust the collimator (e.g. reverse telescope) to produce what appears to be a parallel beam or collimated beam of light. The diameter of the beam can be measured at various distances from the collimator and compared to that predicted by Raleigh-Gaussian beam propagation theory.
Another way is to pass the beam into a distance simulating apparatus. This is composed of a collimator or beam expander followed by a telescope. The light that is being measured for collimation can be input to this apparatus and the output can be made to simulate the spot size that would be obtained at some desired range from the input beam in question. This type of apparatus requires diffraction limited optics throughout and is quite expensive.
A number of printed articles have been located which relate to this subject. These are: Self-referencing Collimation Testing Techniques; K. V. Sriram; M. P. Kothiyal; R. S. Sirohi; January 1998, Vol. 32, No. 1, Optical Engineering.
This paper describes a number of dual-field grating types of shearing interferometers. In particular, the paper describes a double-wedge plate shearing interferometer used for collimation testing, with the fringe pattern in one plate providing a reference for the other. It recognizes that use of two round wedges results in wide separation between the two fields and makes for difficulty in judging the collimation position. As a result the authors developed a double-wedge plate as a single optical component. Using a single elongated source wedge plate (FIG. 10a) in which the wedge direction is parallel to the greater (elongated) direction, that plate has only long edge DC cut and polished, then the plate is cut in two cross-wise, bisecting the polished edge, and the resultant two polished edges CF and FD are joined. Thus, the wedge direction of the two halves of the resulting optical element are opposite (see FIG. 10B).
In order to secure essentially the same fringe patterns (for accurate comparison) it is necessary that the number of fringes and their spacing be uniform over the length of the original or source plate (FIG. 10a). This introduces stringent requirements into the manufacture of a double-wedge plate, both as to the supply and/or selection of the original or source plate, and as to the splitting and joining of the single source plate into a double-wedge plate which is capable of producing closely matched images for accurate and easily used adjacent images.
2. The Use of a Single Plane Parallel Plate as a Lateral Shearing Interferometer with a Visible Gas Laser Source; (M. V. R. K. Murty; April 1964/Vol. 3, No. 4/ Applied Optics); PA0 3. Double Wedge Plate Shearing Interferometer for Collimation Test; Bajpal S. Sirohi and Mahendra P. Kothiyai; (Applied Optics/Vol. 26, No. 19/Oct. 1, 1987). PA0 4. Rotatable Single Wedge Plate Shearing Interference Technique for Collimation Testing; De-Yan Xu, K. J. Rosenbruch; (Optical Engineering; /April 1991/Vol 30 No. 4); PA0 5. Half-Aperture Shearing Interferometer for Collimation Testing; Yun Woo Lee, Hyun Mo Cho, In Von Lee; (Optical Engineering--November 1993/Vol. 32 No. 11);
A high-intensity interference pattern can be obtained in a shearing interferometer with the use of a visible gas laser and a simple construction. The high intensity of the laser per unit of solid angle gives an interference pattern that is visible in room light. The narrow spectral width of the source allows a simple plane parallel plate to be used to obtain the desired shear.
This paper describes a means of comparing fringe patterns from two separate shear plates where the wedge angles are perpendicular to the reflected light. The comparative fringe patterns then rotate with respect to each other, but do not change size (line space frequency) with respect to each other.
Describes the basic theory and technique of a collimation testing method based on analysis of the lateral shearing interference fringes. The changing fringe spacing and the orientation after rotation of a single wedge plate through 180 degrees indicate the degree of collimation of the light beam. The preliminary experimental equipment is described, precision analyses are presented, and the usefulness for practical applications is demonstrated.
A half-aperture shearing interferometer for collimation testing is proposed. It consists of a wedge plate, two plane mirrors, and two baffles. Detailed analyses for three configurations of the system are presented.
Also, U.S. Pat. No. 4,756,175 issued Nov. 22, 1988, discloses a rotatable Shear-Plate Interferometer employing a rotatable shear plate mounted within a tubular support at a 45 degree angle to the center axis of the support. The tubular supporting member is itself rotatably mounted about its center axis and a collimated laser beam can be directed incident to the shear plate along the center axis and the plate rotated defocus of the beam in different directions can be measured.